atlas: fix generic options

Build atlas with the generic options recommended by the upstream
documentation for distributions. The expression now takes the parameter
'threads' which configures the number of threads atlas will use. The
default is to build serial atlas ('threads = "0"'). The expression also
takes the parameter 'cacheEdge' which is the L2 cache per core, in
bytes. This reduces build time because the cache size doesn't need to be
detected. It also reduces impurity, since different build nodes on Hydra
may have different hardware. It is set to 256k by default, which is
recommended for distributions by the upstream documentation.